This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions with treated or preconditioned coated carrier particles prepared, for example, by a dry powder process. In one embodiment of the present invention, the carrier particles are comprised of a core with coating thereover generated from a mixture of polymers that are not in close proximity thereto in the triboelectric series, which coating contains a charge enhancing additive such as distearyl dimethyl ammonium methyl sulfate, reference U.S. Pat. No. 4,560,635, the disclosure of which is totally incorporated herein by reference. Moreover, in another aspect of the present invention the carrier particles are prepared by a dry coating process wherein a polymer or a mixture of polymers are applied to a carrier core enabling insulating particles with relatively constant conductivity parameters; and also wherein the triboelectric charge on the carrier can vary significantly depending on the coatings selected, which coatings are preconditioned with a charge enhancing additive by the mixing thereof with the coating or coatings selected. Developer compositions comprised of the preconditioned carrier particles of the present invention are useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging processes. Additionally, developer compositions comprised of substantially insulating carrier particles prepared in accordance with the process of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core. Also, with the preconditioned carriers the admix characteristics of the developer, especially the toner selected is, for example, from about 15 to about 60, and preferably from about 15 to about 30 seconds. Further changes in At, that is the triboelectric values of the carrier particles are substantially constant. Further, stable triboelectic charge characteristics and admix times for added substantially uncharged toner is less than 60 seconds in many embodiments of the present invention.
The aforementioned A.sub.t refers to the tribo (toner concentration +1) and with the present invention it is this A.sub.t that is relatively constant in most embodiments thus enabling a number of advantages with the developer composition of the present invention. In one embodiment, the A.sub.t for the developers of the present invention can be from about 55 to about 115 and preferably is from about 90 to about 95, reference FIG. 3, for example.
The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that enable continued constant developed images of high quality and excellent resolution, and the admix characteristics together with a stable A.sub.t. The aforementioned characteristics are achieved with the present invention.
In a patentability search report, the following prior art, all U.S. patents, were recited: U.S. Pat. No. 4,803,017 directed to new quaternary ammonium salts as charge control agents in electrophotographic toners and developers, see the Abstract of the Disclosure, for example, and wherein it is disclosed, reference column 2 for example, beginning at line 60, that additionally some of the known quaternary ammonium salt charge agents would adversely interact chemically and/or physically with other developer or copier components, for example, some interact with carrier or carrier coating materials; U.S. Pat. No. 4,812,378 directed to toners with charge control agents of quaternary ammonium salts, and also note column 2, beginning at line 62, wherein it is indicated that additionally some of the known quaternary ammonium salt charge control agents will adversely interact chemically and/or physically with other developer copier components, for example some interact with carrier or carrier coating material; similar teachings are present in U.S. Pat. Nos. 4,812,380 and 4,812,382; 4,726,994 directed to a method of modifying the triboelectric charging propensity of carrier particles coated with a fluorohydrocarbon polymer, which comprises dehydrofluorinating and oxidizing the polymer by contacting the coated particles with a basic solution and the solution of an oxidizing agent, reference for example the Abstract of the Disclosure, with a similar teaching being present in U.S. Pat. Nos. 4,737,435; 4,672,016 directed to carrier particles with a carrier core containing a silicone resin layer thereon, the silicone resin layer containing an organic tin compound and finely divided electroconductive particles, reference the Abstract of the Disclosure and note the teachings in columns 1, 2, 3, 4 and 5; and as background or collateral interest U.S. Pat. No. 3,985,663 directed to conductive inks containing quaternary ammonium compounds, see for example the Abstract of the Disclosure; and U.S. Pat. No. 4,560,635 directed to toner compositions with ammonium sulfate charge enhancing additives, reference for example the Abstract of the Disclosure.
Additionally, carrier particles for use in the development of electrostatic latent images are described in many patents including, for example U.S. Pat. No. 3,590,000. These carrier particles may consist of various cores, including steel, with a coating thereover of fluoropolymers; and terpolymers of styrene, methacrylate, and silane compounds. Recent efforts have focused on the attainment of coatings for carrier particles for the purpose of improving development quality; and also to permit particles that can be recycled, and that do not adversely effect the imaging member in any substantial manner. Some of the present coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have and adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of 20 minutes to about 120 minutes, enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics; and further with regard to the '387 patent, in many situations carrier and developer mixtures with only specific triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated. With the invention of the present application, in an embodiment thereof the conductivity of the resulting carrier particles are substantially constant, and moreover the triboelectric values can be selected to vary significantly, for example, from less than -15 microcoulombs per gram to greater than -70 microcoulombs per gram, depending on the polymer mixture selected for affecting the coating process.
Other patents of interest include U.S. Pat. No. 3,939,086, which teaches steel carrier beads with polyethylene coatings, see column 6; U.S. Pat. No. 4,264,697, which discloses dry coating and fusing processes; U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935 and 4,434,220. Also, it is known to treat carrier particles with metal salts of fatty acids such as zinc stearate to permit, for example, improved flowability of the toner polymer present on the surface of the carrier.
With further reference to the prior art, carriers obtained by applying insulating resinous coatings to porous metallic carrier cores using solution coating techniques are undesirable from many viewpoints. For example, the coating material will usually reside in the pores of the carrier cores, rather than at the surfaces thereof; and therefore is not available for triboelectric charging when the coated carrier particles are mixed with finely divided toner particles. Attempts to resolve this problem by increasing the carrier coating weights, for example, to as much as 3 percent or greater to provide an effective triboelectric coating to the carrier particles necessarily involves handling excessive quantities of solvents, and further usually these processes result in low product yields. Also, solution coated carrier particles when combined and mixed with finely divided toner particles provide in some instances triboelectric charging values which are too low for many uses. The powder coating process that may be selected for the present invention in an embodiment thereof overcomes or minimizes these disadvantages; enables developer mixtures that are capable of generating high and useful triboelectric charging values with finely divided toner particles; and also wherein the carrier particles are of substantially constant conductivity. Further, when the preconditioned resin coated carrier particles are prepared by the powder coating process of the present invention, the majority of the coating materials are fused to the carrier surface thereby reducing the number of toner impaction sites on the carrier material. Additionally, there can be achieved with the process of the present invention, independent of one another, desirable triboelectric charging characteristics and conductivity values; that is, for example, the triboelectric charging parameter is not dependent on the carrier coating weight as is believed to be the situation with the process of U.S. Pat. No. 4,233,387 wherein an increase in coating weight on the carrier particles may function to also permit an increase in the triboelectric charging characteristics. Specifically, therefore, with the carrier compositions and process of the present invention there can be formulated developers with selected triboelectric charging charging characteristics and/or conductivity values in a number of different combinations.
Thus, for example, there can be formulated in accordance with the invention of the present application developers in an embodiment thereof with conductivities of from about 10.sup.-6 mho (cm).sup.-1 to about 10.sup.-17 mho (cm).sup.-1 as determined in a magnetic brush conducting cell; and triboelectric charging values of from about -8 to a -80 microcoulombs per gram on the carrier particles as determined by the known Faraday Cage technique. Thus, the developers of the present invention can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, maintaining the same coating weight on the carrier particles and changing the polymer coating ratios. Similarly, there can be formulated developer compositions wherein constant triboelectric charging values are achieved and the conductivities are altered by retaining the polymer ratio coating constant and modifying the coating weight for the carrier particles. Also of importance with respect to the present invention is the process of preconditioning the carrier core and/or carrier coating with charge enhancing additive thereby permitting desirable admix characteristics, preferably of from about 15 to about 30 seconds, and stable A.sub.t.
Disclosed in U.S. Pat. Nos. 4,937,166 and 4,935,326, the disclosures of which are totally incorporated herein by reference, are developers containing carrier particles with polymeric coatings not in close proximity in the triboelectric series. More specifically, these patents disclose carrier particles prepared by dry mixing low density porous magnetic or magnetically attractable metal core carrier particles with from, for example, between about 0.05 percent and about 3 percent by weight, based on the weight of the coated carrier particles, of a mixture of polymers until adherence thereof to the carrier core by mechanical impaction or electrostatic attraction; heating the mixture of carrier core particles and polymers to a temperature, for example, of between from about 200.degree. F. to about 550.degree. F. for a period of from about 10 minutes to about 60 minutes enabling the polymers to melt and fuse to the carrier core particles; cooling the coated carrier particles; and thereafter classifying the obtained carrier particles to a desired particle size. In a specific embodiment of the aforementioned copending applications, there are disclosed carrier particles comprised of a core with a coating thereover comprised of a mixture of a first dry polymer component and a second dry polymer component, which are not in close proximity in the triboelectric series. Therefore, the aformentioned carrier compositions, which can be selected for the compositions of the present invention in an embodiment, can be comprised of known core materials including iron with a dry polymer coating mixture thereover. Subsequently, developer compositions can be generated by admixing the aforementioned carrier particles with a toner composition comprised of resin particles and pigment particles. With the carrier of the aforementioned applications, there can be obtained stable triboelectric properties independent of conductivity, and stable conductivity characteristics independent of the triboelectric charge of the toner.